U.S. Pat. Nos. 4,640,352 and 4,886,118 disclose conductive heating of subterranean formations of low permeability that contain oil to recover oil therefrom. Low permeability formations include diatomites and oil shales. Formations of low permeability are not amenable to secondary oil recovery methods such as steam, carbon dioxide, or fire flooding. Flooding materials tend to penetrate formations that have low permeabilities preferentially through fractures. The injected materials bypass most of the formation hydrocarbons. In contrast, conductive heating does not require fluid transport into the formation. Oil within the formation is therefore not bypassed as in a flooding process. When the temperature of a formation is increased by conductive heating, vertical temperature profiles will tend to be relatively uniform because formations generally have relatively uniform thermal conductivities and specific heats. Transportation of hydrocarbons in a thermal conduction process is by pressure drive, vaporization, and thermal expansion of oil and water trapped within the pores of the formation rock. Hydrocarbons migrate through small fractures created by the expansion and vaporization of the oil and water.
When the formation contains high molecular weight oil or hydrocarbon solids, thermal conduction could also result in pyrolysis of the hydrocarbons in-situ. The products of the pyrolysis will be of lower molecular weights and will therefore be more valuable than the original oil. Pyrolysis of solids also creates additional voids within the formation rocks. These voids provide additional hydrocarbon mobility.
Considerable effort has been expended to develop electrical resistance heaters suitable for injecting heat into formations having low permeability. U.S. Pat. Nos. 5,065,818 and 5,060,287 are exemplary of such effort. Electrical heating of formations is relatively expensive compared to directly burning a hydrocarbon fuel. It would be preferable to provide a heat injection method in which directly burns a hydrocarbon fuel.
Gas fueled well heaters that are useful for heating formations to temperatures sufficient for ignition of in-situ fire floods are disclosed in U.S. Pat. Nos. 3,095,031; 3,880,235; 4,079,784; and 4,137,968. Provisions for the return of combustion gases to the 4,137,968. Provisions for the return of combustion gases to the surface are not required because the combustion gases are injected into the formation. The fuel gas and combustion air also remain relatively cool as they go down a borehole toward the burner because combustion gases rising in the borehole do not heat the burner. Additionally, a long service life is not required due to the short time period during which the burner is needed. These burners are therefore not suitable for use as heat injectors and do not overcome the shortcomings of the prior art heat injector burners.
Gas fueled heaters which are intended to be useful for heat injection are disclosed in U.S. Pat. No. 2,902,270 and Swedish Patent No. 123,137. These burners utilize flames to combust fuel gas. The existence of flames cause hot spots within the burner and in the formation surrounding the burner due to radiant heat transfer from the luminous portion of the flame. A typical gas flame provides about a 1650.degree. C. radiant heat source. Materials of construction for the burners must be sufficient to withstand the temperatures of these hot spots. The heaters are therefore more expensive than a comparable heater without flames. The heater of Swedish Patent 123,137 would appear to result in a flameless combustion such as the present invention if the combustion air and the fuel gas were heated to a temperature above the autoignition temperature of the mixture. But due to the shallow depths of the heat injection wells disclosed in that patent, the components do not appear to be heated to this extent by the combustion gases. Further, radiant heat transfer from the flames appears to be critical in obtaining the temperature profile indicated in FIG. 2 of the Swedish patent because little heat is transferred from the well bore to the formation above the borehole containing flames. Due to the existence of flames, the service life and the operating temperatures of these burners are unacceptably limited.
The Swedish patent also addresses the problem of creation of carbon from hydrocarbon gases at elevated temperatures. The carbon is removed by exchanging services between the air and the fuel lines. Any carbon deposited in a line while the line is in fuel service is removed when the line is in combustion air service. This requires that the fuel gas lines be as large as the combustion air lines. Because about ten moles of combustion air ar required for each mole of methane burned, and because combustion air is generally optimally supplied at a lower pressure, a fuel gas line will have to be considerably larger to also accommodate combustion air flow service. The burner would therefore be considerably more expensive than one in which services of these two conduits are not interchangeable.
The Swedish patent also discloses that "before the gas is brought into the gas tube . . . it can through suitable preparation in a was known in the gas technique be given such a composition that it does not deposit coke." It is not clear what this statement means, but it likely is suggesting that hydrocarbons heavier than methane or ethane be removed by cryogenic distillation to reduce the tendency of the gas to form coke.
U.S. Pat. Nos. 3,113,623 and 3,181,613 disclose gas fired heat injection burners for heating subterranean formations. These burners utilize porous materials to hold a flame and thereby spreading the flame out over an extended length. Radiant heat transfer from a flame to the casing is avoided by providing the porous medium to hold the flame. But for combustion to take place in the porous medium, the fuel gas and the combustion air must be premixed. If the premixed fuel gas and combustion air were at a temperature above the autoignition temperature of the mixture, they would react upon being mixed instead of within the porous medium. The formations utilized as examples of these inventions are only up to fifty feet thick and below only about fifteen feet of overburden. The fuel gas and the combustion air are therefore relatively cool when they reach the burner. The burner would not function at it was intended if the formation being heated were significantly deeper.
It is therefore an object of the present invention to provide a method to inject heat into a subterranean formation using a fuel gas combustor which does not require a flame in the borehole during the heating process. It is a further object to provide such a method which does not require complicated equipment within the borehole. It is another object of the present invention to provide a method which has a high level of thermal efficiency.